1. Field of the Invention
The present invention relates to a ceramic heater for use in, for example, a glow plug, and to a method for manufacturing the same.
2. Description of the Related Art
A conventionally known ceramic heater for use in, for example, a ceramic glow plug is configured such that a resistance-heating member formed of a conductive ceramic material or a like material is embedded in an electrically insulative ceramic substrate. A ceramic substrate formed of silicon nitride ceramic is widely used, by virtue of its excellent thermal shock resistance and high-temperature strength.
In the course of manufacture of a ceramic heater using the above-mentioned ceramic substrate, in many cases, a green body which is to become a ceramic substrate, is fired in order to enhance mechanical strength. However, in some cases of normal firing, differences in thermal expansion coefficient and sintering properties between silicon nitride ceramic and a conductive ceramic material raise a problem such as cracking in a boundary portion between the materials. Therefore, firing through hot pressing, which is conducted under a predetermined pressure, is often employed.
Firing through hot pressing employs a carbon jig for applying pressure to the above-mentioned green body with a parting agent such as BN present therebetween. A problem arises in this firing process, since silicon contained in silicon nitride ceramic and carbon contained in the carbon jig react with each other to produce silicon carbide. For example, since firing is performed in a reducing atmosphere induced by carbon, an oxide used as a sintering aid for silicon nitride tends to move toward the surface of silicon nitride ceramic in the course of firing. Therefore, in some cases, an uneven composition arises, thereby causing partial impairment in strength. In the case where a rare-earth oxide is used as a sintering aid, the melilite crystal phase is apt to be generated from firing in the ceramic substrate. In some cases, the melilite crystal phase induces low-temperature oxidation at around 1000° C., leading to cracking in the ceramic substrate (ceramic heater).
Further, silicon contained in silicon nitride and carbon contained in the carbon jig react with each other to produce silicon carbide, thereby raising the following problem. For example, defective firing of the surface of silicon nitride ceramic may occur, thereby causing impairment in strength. Also, the reaction of silicon nitride and carbon in the course of firing may induce bonding between silicon nitride ceramic and the carbon jig, which have different thermal expansion coefficients; thus, in subsequent cooling, the bonding may cause cracking in the carbon jig. Additionally, the carbon jig is apt to ablate through oxidation, thereby shortening the life thereof. 